Railroad signal relays have been in use for 100 years or more. Many installations in use today have been in service for over sixty years. Typically, signal relays are installed in bungalows or field cases alongside the tracks. As these enclosures are neither heated nor air-conditioned, the relays are thus exposed to temperature extremes as well as vibration from passing trains. Therefore, rugged design is essential.
Furthermore, safety and fail-safe operation concerns are of prime importance in the design and operation of railroad signaling systems. To meet the fail-safe requirements, most relays are of the gravity type, meaning that the armature falls to a de-energized position under the force of gravity when the coil is not energized. An energized coil overcomes the gravity force and picks up the armature. Both normally open and normally closed contacts are employed. Contacts are relatively massive due to current carrying requirements and exposure to current and voltage surges from lightning strikes and other transients.
As part of the fail-safe design of vital circuits, many relays are normally in the energized, or picked, state. Any interruption of power in the coil circuit, whether due to power supply failure or a signal wire break, causes the relay to open. This results in a more restrictive and safer condition.
Gravity relays are mounted in a particular orientation to work properly, and are typically designed as either shelf-mounted or rack-mounted. Shelf-mounted relays are usually mounted on shock mounts fastened to a shelf within a wayside bungalow. Connection wires with soldered or crimped ring terminals are bolted to studs on top of the relay. Shelf relays are individually installed along many shelves.
Rack mounted relays are designed to interface with standard blocks mounted on a vertical frame in horizontal rows. Blocks come in different sizes depending upon relay type and relay manufacturer. A commonly used block system uses two size blocks. The B-1 block has a width of 2.5 inches while the B-2 block has a width of 5 inches. The mounting design and hardware for these blocks is identical, allowing them to be intermixed in rows according to the design requirements of the signaling system.
Connections from the field wiring are made to the rear of the B-1 or B-2 blocks. Connections are either fastened to studs using ring terminals or inserted into slots using special crimped terminals with retaining clips. These terminals may be removed using a special tool and re-inserted into another connection slot as desired.
A B-1 or B-2 type relay is mounted to the front of the B-1 or B-2 block. The mating surface of the relay has spring contacts that mate with the wiring terminals inserted in the slots from the rear. Two rods are used as guides for the relay during insertion. These rods are threaded on the ends and are further used to firmly retain the relay in place with knurled nuts and lock nuts. Other mounting and retention designs by other manufacturers perform the same functions using slightly different mechanisms.
On the most commonly used relay block system, the coil connections are brought out to the front of the block below the relay. Two stud connections are provided. One connection (called the 1E post) provides the low side or DC return side of the circuit. The other connection (called the 3E post for B-1 relays and the 5E post for B-2 relays) provides the control signal or energizing side of the circuit. This connection is made through a special stud with a nonconductive shoulder that isolates a ring terminal placed around the shouldered stud from the stud itself. A barrel nut is screwed down over the stud to make contact with the ring terminal and thereby connect the control signal to the high side of the coil. The barrel nut may be opened (unscrewed) to disconnect the circuit from the relay for testing purposes without removing any wires, and is frequently used by maintenance personnel to drop a given relay.
Although the barrel nut may be loosened to drop a particular relay, there is no easy way to test an individual relay in a rack. Due to the safety issues involved in railroad signaling, regulatory requirements dictate periodic testing of relays employed in vital signaling circuits. These tests are typically performed on a two-year cycle.
Testing requires that a relay first have its coil saturated. The coil current is then slowly decreased until the relay drops. The current at which the drop occurs is recorded as the drop current. The current is further reduced to zero and then slowly increased until the relay picks up. This current is recorded as the pick current. Close observation of the armature is required to determine the drop and pick points. Careful and slow increase and decrease of the coil current is necessary in order to obtain accurate and repeatable readings of pick-up and drop-away currents.
Prior art as practiced in some Union Switch and Signal relays incorporates a pair of test contacts on the front surface of the installed relay. These contacts are placed in series with the coil circuit and are normally closed to maintain a circuit through the coils. A test probe may be inserted that separates the contacts and opens the coil circuit. At the same time, conductive surfaces on each side of the test probe each make contact with one of the contacts. Two conductive leads from the probe provide the ability to connect test equipment to the circuit or to the coil as required. A separate connection must be made by another means to the low side of the coil (or DC return) to complete a test circuit through the relay coil.